<html>
  <body>
    <form runat="server">
      <asp:TextBox ID="op1" RunAt="server" />
      +
      <asp:TextBox ID="op2" RunAt="server" />
      <asp:Button Text="  =  " OnClick="OnAdd" RunAt="server" />
      <asp:Label ID="Sum" RunAt="server" />
    </form>
  </body>
</html>

<script language="C#" runat="server">
  void OnAdd (Object sender, EventArgs e)
  {
      int a = Convert.ToInt32 (op1.Text);
      int b = Convert.ToInt32 (op2.Text);
      Sum.Text = (a + b).ToString ();
  }
</script>